Utility knives typically have a sharp cutting blade that can either (a) be retracted into a housing, or (b) released to an operating disposition by movement of a protective blade guard. In either case problems arise where the blade is left in an unprotected disposition where it can accidentally cause injury to a user.
The problem of accidental injury has been long recognized, with numerous solutions being put forward at various times. U.S. Pat. No. 4,980,977 to Matin et al. (January 1991), for example, describes a knife having a safety guard that guards the blade when not in use, and automatically retracts as the blade is removed from the workpiece. The guard has a manually triggered self-locking release assembly that automatically relocks the guard when retracted. Unfortunately, Matin's locking mechanism is external to the housing housing, which is dangerous because the mechanism is readily subjected to debris that could jam or otherwise interfere with both the locking and unlocking functions. In addition, Matin's safety guard pivots off the blade externally to the housing housing, rather than being retracted into the housing. That operation is dangerous because the pivoted guard can readily interfere with operation of the knife.
U.S. Pat. No. 5,878,501 to Owens et al. (March 1999) uses an internal locking mechanism, but leaves the blade in the “use” position for multiple uses. There is no automatic re-locking mechanism, and withdrawal of the blade into the housing is entirely manual.
More recently the present inventor pioneered utility knives having a mechanism that automatically re-locks the protective blade guarding to prevent more than a single use of the blade. Pending applications include Ser. No. 09/804,451, published in September 2002 as 2003/0131393, and Ser. No. 10/300,382, published in May 2004 as 2004/0093734. These and all other referenced patents and applications are incorporated herein by reference in their entirety.
While providing considerable improvement over the prior art, the preferred embodiments of the utility knives described in the Ser. Nos. 09/804,451 and 10/300,382 applications have more “play” in the blade guard than might be desired in some circumstances. In the Ser. No. 10/300,382 application, for example, a preferred locking mechanism utilizes a pawl that rides in a looped pathway. Two ramped steps on the pathway limit the pawl's travel to a one-way direction, so that once the pawl starts along the pathway, it must finish a complete loop. The mechanism, however, allows some slight backward motion of the pawl, and thus introduces potentially undesirable play in the blade guard.
Thus, there is a need for an improved locking/releasing mechanism that automatically re-locks the protective blade guarding to prevent more than a single use of the blade, while reducing the play in the blade guard.